1. Field of the Invention
The invention relates to a backlight unit, and more particularly, to a backlight unit provided with a light emitting diode (LED) having a cooling structure.
2. Description of the Related Art
Generally, a cathode ray tube (CRT) is one of the displays usable as a monitor of a television, a meter, an information terminal or the like. However, CRT technology has failed to actively cope with the miniaturization and reduced weight requirements of electronic appliances due to the weight and volume of CRTs. In other words, the CRT is limited by its weight and volume, which is disadvantageous to the trend toward miniaturization and reduced weight of electronic appliances.
To solve the drawbacks of CRTs, flat panel displays have emerged, which include a liquid crystal display device (LCD) using the electro-optic properties of liquid crystal, a plasma display panel (PDP) using gas discharge, an electroluminescence display (ELD) using the electro-luminescence effect, etc.
Since the LCD has advantages such as a slim profile, light weight and low power consumption, it finds utilization as a monitor of a laptop computer, a monitor of a desktop computer, a large-sized information display and the like.
Since most LCDs are light receiving devices that display an image by adjusting the amount of light that is incident from the outside, they essentially require a light source that projects light to the LCD panel, i.e., a backlight.
The backlights used as light sources for LCDs are classified into edge type and direct type depending on where a cylindrical fluorescent lamp is located.
The edge type backlight includes a lamp unit disposed at a side of a light guide plate for guiding light. The lamp unit includes a lamp for emitting light, and a lamp holder is inserted into both ends of the lamp to protect the lamp. A lamp reflector encloses an outer circumferential surface of the lamp and a side portion inserted into a side portion of the light guide plate, for reflecting the light of the lamp toward the light guide plate.
The edge type backlights are employed in small-sized LCDs such as a monitor of a laptop computer or a monitor of a desktop computer.
Also, the direct type backlight has been actively developed as the size of the LCD increases to 20 inches or more. In the direct type LCD, multiple lamps are arranged below a diffusion plate to project light toward a front surface of the LCD panel.
Since the direct type backlight has a higher light efficiency than the edge type backlight, it is mainly used in a large-sized LCD requiring high brightness.
However, the LCD employing the direct type backlight is generally used in a large-sized monitor, i.e., a television or the like, and its usage time is significantly long. Also, the LCD employing the direct type backlight uses a number of lamps, and the lamps may be disordered or lose durability, so that they may not turn on.
Also, an LCD can employ the edge type backlight in which lamp units are respectively installed at both sides of the light guide plate. In this configuration, if one lamp fails and does not turn on, the brightness of the LCD decreases, but a serious problem does not result.
However, an LCD employing the direct type backlight has multiple lamps installed below the LCD panel, and if one lamp does not turn on due to the loss of durability or failure, a portion where the lamp is not turned on becomes noticeably dark compared with other portions. Thus the portion where the lamp is not turned on is a visible defect on the display.
Also, the backlight includes a drawback arising from light efficiency being lowered due to disassembly and assembly of the fluorescent lamps or heat generation.
Recent trends require that the direct type backlight unit as well as the edge type backlight unit be miniaturized, be slimmed down and have low weight. To meet these requirements, the related art backlight unit employs a light emitting diode (LED) that has the advantages of good power consumption, weight, brightness and the like.
FIGS. 1A and 1B are plane and sectional views illustrating a direct type backlight unit provided with an LED.
FIGS. 1A and 1B show a direct type backlight unit that includes multiple LEDs 10 serving as a fluorescent lamp, a printed circuit board (PCB) 12 disposed below the LEDs 10, a bottom cover 14 on which the PCB 12 is mounted, and optical sheets 16 including a diffusion sheet for diffusing the light generated by the LEDs 10.
An LCD panel 18 is disposed above the backlight unit as shown in FIG. 1B, and the LCD panel 18 and the backlight unit are supported by a guide panel 17 and are fixed by a top case 19.
The LEDs 10 include sequentially arranged red LEDs for generating red light, green LEDs for generating green light, and blue LEDs for generating blue light. The PCB 12 includes a circuit for controlling the LEDs 10. Also, the PCB 12 supports the LEDs 10 and simultaneously controls light emission of the LEDs 10 using the circuit for controlling the LEDs 10. The optical sheets 16 including the diffusion sheet are spaced apart by a predetermined distance from the LEDs 10 such that the different lights generated by the LEDs 10 distribute uniformly.
As shown in FIG. 1A, the LEDs 10 include red (R), green (G) and blue (B) LEDs, that are arranged sequentially and repeatedly. For Example, the LEDs 10 are turned on or off in the order of R of 1st line→R of 2nd line→R of 3rd line→G of 1st line→G of 2nd line→G of 3rd line→B of 1st line→B of 2nd line→B of 3rd line. However, other sequences can be used.
Since the LEDs have a lower light efficiency than the related art CCFLs (cold cathode fluorescent lamps), they consume much more power than the related art CCFLs, which results in the generation of much heat.
As can be seen from the structure depicted in FIG. 1B, in the related art LCD, most of heat generated by the LEDs diffuses to the bottom cover 14 disposed at the rear of the LCD, and the heat is then radiated. In a real measurement, the bottom cover 14 shows a temperature range of 80 to 90° C., which is due to the insufficient heat radiation capacity of the bottom cover 14.
This 80 to 90° C. temperature range is higher by about 40° C. than the temperature range of the backlight unit provided with CCFLs, and serves as a serious drawback by causing deformation of the optical sheets, by destroying LED junctions, by decreasing the light efficiency of LEDs, and by degenerating the liquid crystal material to seriously decrease the reliability of the main parts of the display.